In recent airplanes, electrical power requirements are becoming ever greater. Ever more powerful electricity generators are being used. Such an electricity generator is driven by a turbojet of the airplane. It is known to take off the required mechanical power from the shaft that is associated with the high pressure turbine. More particularly, a radial shaft is mechanically coupled to the axial shaft associated with said high pressure turbine, and said power is delivered to a gearbox that drives the electricity generator. When a large amount of electrical power is required, that solution gives rise to difficulties when the engine is idling, whether on the ground or in flight, because of the high level of torque required to drive the electricity generator. This torque tends to cause the high pressure turbine to operate in an undesirable operating range.
The invention seeks to improve that known system in order to take account of recent requirements in terms of auxiliary power, and consequently in order to take account of the new levels of torque that need to be delivered without affecting the operation of the high pressure turbine while idling.
The idea on which the invention is based consists, while idling, in degrading the efficiency of the low pressure turbine situated downstream from the high pressure turbine so as to make it possible to increase the speed of rotation of the high pressure turbine (to a level that enables a high level of power to be taken off therefrom) without causing an increase in the thrust from the engine, where such an increase is not desirable during stages when the engine is idling.